Take strokes with Nothing meta, and put them into the next Meta.
The Nothings were added by realize_group out of Solkattu._dropped, so
this is putting them back into their groups after being converted to
strokes.

format text

The line breaking for rulers is a bit weird in that if the line is broken,
I only emit the first part of the ruler. Otherwise I'd have to have
a multiple line ruler too, which might be too much clutter. I'll have to
see how it works out in practice.